It. A. Daly — Mechanics of Igneous Intrusion. 271 



second view doubtless appeals the more strongly to the majority 

 of those geologists who have actually to do with granitic bodies 

 in the field. In fact, the impression has prevailed among some 

 of them that the " laccolithic theory " is as widely held as it 

 is because of its apparent necessity in the prevailing theory of 

 rock-differentiation. Yet it must be considered as conclusively 

 proved for the great majority of stocks and batholiths inves- 

 tigated, that analysis has not yet shown that the second or 

 " assimilation " theory really meets its own crucial test, the 

 chemical and mineralogical blood-relationship between the 

 average intrusive rock and its country-rock along their mutual 

 contact. Currents within the magma would, of course, tend 

 to remove and diffuse products of assimilation from molar con- 

 tacts ;* but it is extremely doubtful that they could so com- 

 pletely mask the expected results of the process as is over and 

 over again illustrated in nature. No single fact concerning 

 granite, for example, is more striking than its astonishing 

 homogeneity in contact with argillite, limestone, crystalline 

 schist or basic igneous formation — a homogeneity that persists, 

 too, from contact to center of the eruptive. In the very com- 

 mon case where the assimilated product is more acid than the 

 original magma, it would tend to rise through the latter, 

 slowly diffusing in the journey. The upper part of the magma 

 basin should, for that reason, become filled with a mixed magma 

 more siliceous than the original. Heterogeneity, even stronger 

 vertically than horizontally, would be expected in a diorite or 

 gabbro magma cutting crystalline schists, or in a granite magma 

 cutting heavy beds of sandstone or quartzite. True thermal 

 convection currents must, under these conditions, be greatly 

 weakened by the strong differences in density of the original 

 magma and the magma diluted, so to speak, by more siliceous 

 material. In the absence, then, of the only kind of current 

 likely to be set up in the process of cooling and mere caustic 

 solution on molar contacts, the diffusion of the diluted magma 

 would take place only with extreme slowness, f Yet, up to 

 the present time, this consequence of considerable vertical 

 heterogeneity under the stated conditions has not been demon- 

 strated in nature. The recorded field discoveries point, on the 

 contrary, to a distinct failure of the known facts to match the 

 deduction from the theory. The few pi'oved instances of endo- 

 morphic changes of magmas by assimilation (e. g., the granite of 

 the Pyrenees described by Lacroix) serve, by their conspicuous 



*For ease of reference to the great surfaces of contact between intrusive 

 bodies and the terranes cut by them, as distinguished from the contacts of 

 the corresponding igneous rocks and foreign fragments caught in them, the 

 author again introduces a term not in general use but convenient for tem- 

 porary employment. The former are called " molar contacts." 



f Cf . Becker, this Journal (4), iii, p. 30, 1897. 



